In an absolute time simulcast broadcast, if a remote base station were to lose synchronization (e.g., brown out) with the other base stations, the base station could restart broadcasting only if its baseband signal is in sync with the other base stations' baseband signals. If the remote could not resync upon restarting, the remote would have to stay dekeyed for the duration of the simulcast broadcast for fear of destructively interfering with the other transmissions that were synchronized.
Referring now to FIG. 1, there is shown a first time diagram comparing the carrier signals of several remotes. There is shown the carrier signal A.sub.1 of a first remote, the carrier signal A.sub.2 of a second remote, and the carrier signal A.sub.3 of a third remote. In the first time interval 141 the three carrier signals are aligned in time over the optimal coverage point. In the second time interval 143, the carrier signal A.sub.1 of the first remote, fails, that is, suffers a brown out. In the third time interval 145, and subsequent to the brown out suffered by the carrier signal A.sub.1 of the first remote, it is assumed that the first carrier awakens, begins transmitting without synchronization or phase alignment, and destructively interferes with the second remote signal A.sub.2, and the third remote signal A.sub.3, because it is out of phase. Refer to the corresponding sinusoidal waveforms 115, 125 and 135. One method of achieving phase alignment, of course, is for each remote site to begin propagation of its signal based on a predetermined launch time.
If it were to broadcast without being synchronized, it would degrade the simulcast broadcast.
Another possibility, of course, would be for the remote site to cease broadcasting in the event it loses the needed phase control. This is shown in FIG. 2. Referring now to FIG. 1, there is shown a second time diagram comparing the carrier signals of several remotes. There is again shown the carrier signal A.sub.1 of a first remote, the carrier signal A.sub.2 of a second remote, and the carrier signal A.sub.3 of a third remote. In the first time interval 241 the three carrier signals are aligned in time over the optimal coverage point. In the second time interval 243, the carrier signal A.sub.1 of the first remote fails, that is, suffers a brown out. In the third time interval 245, and subsequent to the brown out suffered by the carrier signal A.sub.1 of the first remote, it is assumed that the first carrier awakens, but ceases transmitting because it is unable to recover necessary synchronization or phase alignment. Although no destructive interference results, the coverage area is compromised because of one less simulcast signal. The cost of losing a base station during a simulcast broadcast is related to the loss of coverage area.
What is needed, of course, is a method and apparatus for synchronization that will achieve signal timing shown in FIG. 3. In this figure, as in the previous FIGS. 1-2, it is assumed the carrier signal A.sub.1 of the first remote fails during time interval 343. In this case, however, during the subsequent time interval 345, as depicted in FIG. 3, after the brown out, the first remote awakens, receives the realignment information, and re-enters simulcast transmission in phase with the other remotes. As a result, the corresponding sinusoidal signals 315, 325, and 335 are correctly aligned with respect to time and phase.
Therefore, there is a need for an improved synchronization method and apparatus.